1. Field of the Invention
The present invention relates to the exercise of control over gas turbine engines and particularly to controlling the rate of delivery of fuel to the gas generator of a free turbine engine employed as the power plant for a rotary wing aircraft. More specifically, this invention is directed to a closed loop gas turbine engine fuel control wherein fuel flow is varied as a function of the ratio of the rate of change of gas generator speed to compressor discharge pressure. Accordingly, the general objects of the present invention are to provide novel and improved methods and apparatus of such character.
2. Description of the Prior Art
Gas turbine engines are subject to the operating condition known as "surge", i.e., a mismatch in the speed of the compressor blades and the incoming air. When the surge condition occurs there is a large loss of power, a loss of air flow, an increase in temperature and substantial mechanical vibration. The surge condition is usually encountered during engine acceleration when the rate of delivery of fuel to the gas generator becomes excessive.
Prior art gas turbine engine controls may be generally characterized as being of either the "open" or "closed" loop type. Open loop controls are scheduling devices wherein the fuel flow to the engine is varied as a function of speed. Thus, open loop gas turbine engine controls are insensitive to changes in the fuel control itself, changes in the engine or changes in the characteristics of the fuel being supplied to the engine. Prior closed loop gas turbine engine fuel controls typically operate in a mode wherein the rate of change of the gas generator shaft speed is determined and fuel flow to the engine is varied so as to match schedules devised when the engine was new. Thus, the prior art closed loop control may have a speed governor which, under steady state conditions, demands a fuel flow as a function of gas generator speed. The control will also have an acceleration schedule which will set the fuel flow rate during acceleration. A surge margin will be built into the acceleration schedule. Because of this surge margin, the accelerating engine will be able to accept a predetermined percent of additional fuel flow before it will enter the surge condition. Since the engine and/or fuel control may undergo changes in operating characteristics with extended use, the surge margin may actually decrease with time. If excess fuel is delivered to the engine when it is in the surge condition, for example because the surge margin has decreased and thus the acceleration schedule is actually calling for excess fuel, it is possible that the engine will either stay in surge or be subjected to multiple surges.
Continuing to discuss prior art closed loop gas turbine engine fuel controls, it has been conventional practice to select the appropriate fuel flow as a function of the ratio of the rate of change of gas generator speed, NDOT, to compressor inlet pressure, PT2. While experience has shown that such a closed loop NDOT/PT2 acceleration control results in enhanced performance when compared to an open loop control, during engine surges an NDOT/PT2 control will attempt to increase fuel flow to compensate for the stalled acceleration. This increases the probabililty that the engine will not accelerate through the surge prone area and multiple surges will occur thus requiring pilot intervention.